1. Field of the Invention
Wastewater biomass systems normally consist of a mixture of water, organic matter and a variety of bacterial genera whose food, to a large degree, consists of the organic matter component of the biomass and/or other organic waste materials (e.g., volatile solids). The products of anaerobic digestion of such biomass systems normally consist of: (1) a gas phase primarily comprised of carbon dioxide, methane, ammonia, small amounts of other gases (e.g., hydrogen sulfide and hydrogen) and trace amounts (e.g., less than one tenth of one percent by volume) of certain other gases (e.g., propane), which, in total, constitute what is commonly referred to as xe2x80x9cbiogasxe2x80x9d; (2) a liquid phase (aqueous in nature) in which ammonia, nutrients and a host of organic chemicals and gases and inorganic chemicals are dissolved; and (3) a colloidal phase of suspended solids containing undigested organic and inorganic compounds, synthesized biomass and/or bacterial cells.
Progressive destruction of the organic matter in such biomass systems has been made more efficient by introducing various solubilized nutrients for the bacteria into such systems. Various gases also have been injected into these systems as gas stripping agents. In effect, these gas stripping agents dissolve those biogas molecules (e.g., methane, carbon dioxide, ammonia, hydrogen, hydrogen sulfide, etc.) that are produced as waste products of the bacterial metabolic processes carried out in such biomass systems. These waste product gases are produced within the bacteria cells, permeate their cell walls and collect, in the form of bubbles, on their outer cell wall surfaces.
Nitrogen and hydrogen, obtained from sources outside the biomass system, as well as recirculated biomass product gases (e.g., carbon dioxide, methane and hydrogen), have been used to strip waste product gas bubbles from the bacterial cell walls. Again, the waste product gas molecules on the outer surfaces of the cells are dissolved in the stripper gases. Consequently, newly produced waste product gas molecules are able to leave the outer surfaces of the bacteria cell walls more quicklyxe2x80x94and thereby produce more efficient biomass digestive processes.
2. Description of Related Art
Many academic, patent and trade publications have recognized the above-noted biomass gas inhibition problems and have suggested various ways to solve them. For example, within the academic literature, the following articles give some particularly good insights into the nature of these problems. Finney, C. D., and R. S. Evans, Anaerobic Digestionxe2x80x94the Rate Limiting Process and the Nature of Inhibition, Science, 1975, Vol. 190, p. 1088; McCarty, P. L., Anaerobic Waste Treatment Fundamentals, Part I, Public Works, 1964, p. 107; and Obayashi, A. W., and J. M. Gorgan, Management of Industrial Pollutants by Anaerobic Processes in Industrial Waste Management Series, W. James (ed.), Lewis Publishers, Inc., Chelsea, Mich., 1985; and Anaerobic Digestion Processes in Industrial Wastewater Treatment, Stronach, S. M., Rudd, T., and Lester, S. N., Springer-Vertog, Berlin, 1986.
The Finney and Evans article, for example, postulates that the rate controlling step in anaerobic digestion processes of this kind may be the fact that the cells"" gaseous waste products (e.g., CH4+CO2) must eventually undergo a transfer from the system""s liquid phase to its gas phase. That is to say that, even though many previous investigators had considered the biological conversion of organic acids (such as acetic acid and propionic acid) into methane and carbon dioxide as being the overall rate controlling reactions, Finney and Evans postulated other rate limiting steps. These authors suggested that the rate of removal of gas bubbles away from bacterial cell walls might constitute the rate limiting step.
Finney and Evans also postulated that, by remaining attached to their cell walls, waste product gas bubbles effectively (a) decrease the cell wall""s gas transfer, surface area, (b) decrease the cell wall/gas layer system""s overall permeability and (c) reduce the cell""s ability to absorb needed nutrients. They also considered the possibility that the presence of certain toxic compounds in the biomass may interfere with a bacterial cell walls"" ability to pass dissolved nutrients into the cells.
Many patent references teach various specific processes for stripping gases, such as hydrogen sulfide, from anaerobic cells in digester systems. U.S. Pat. No. 5,651,890 (xe2x80x9cthe ""890 patentxe2x80x9d, to applicant) is a particularly relevant patent in this regard; hence, its teachings are incorporated herein by reference. The ""890 patent teaches that anaerobic digestion of wastewaters can be made more effective by introducing propane gas (from an external source) into the biomass in order to strip bacterial waste product gases such as H2S from bacteria cell walls. This patent does not however teach or suggest (1) the beneficial effects of stripping CH4 and CO2 gases off of the volatile solids component of the biomass system (and thereby creating more surface area on the volatile solids to which waste product gases can adhere) nor (2) the reduction of H2S produced in the biogas. Moreover, the ""890 patent does not teach or suggest intermittent introduction of propane gas into a biomass in order to make such processes more efficient. Indeed, the ""890 patent (see col. 11, line 14) teaches away from the concept of introducing propane intermittently. It also should be noted that the ""890 patent also very strongly teaches away from the concept of using propane as a bacterial nutrient (see col. 13, lines 33-53) or having propane interact in any other biological process.
U.S. Pat. No. 4,289,625 (the ""625 patent) teaches use of a hybrid, bio-thermal system comprised of an anaerobic digester unit and a thermal gasifier unit. In effect, the anaerobic digester system of the ""625 patent achieves greater methane production per unit of feed by xe2x80x9cdigestingxe2x80x9d and xe2x80x9ccrackingxe2x80x9d the anaerobic sludge material and, secondarily, by feeding the thermal gasifier""s, gaseous products back to the digester unit as food sources for the microorganisms residing therein. Some of these gaseous products are characterized as xe2x80x9cCxHyxe2x80x9d in the ""625 patent, but no particular emphasis is laid upon a propane gas component that may fall within the generalized term xe2x80x9cCxHyxe2x80x9d. This is not surprising because a thermal gasifier such as this would tend to produce an extremely varied source of hydrocarbons due to its xe2x80x9ccrackingxe2x80x9d ability.
U.S. Pat. No. 5,298,163 teaches that a xe2x80x9cneutralxe2x80x9d gas (no disclosures are made as to the exact identity of such a xe2x80x9cneutralxe2x80x9d gas) can be introduced in a biodegration process in order to strip or otherwise displace hydrogen sulfide gas from a biomass system but no H2S reduction in the produced biogas was noted.
U.S. Pat. No. 5,015,384 teaches an anaerobic digestion process that injects xe2x80x9canoxicxe2x80x9d gases to strip carbon dioxide gas from a biomass system so that its pH remains neutral or nearly so. This reference states: xe2x80x9cThe primary requirement is that the gas be anoxic, i.e., not contain oxygen or other constituents toxic to the anaerobic bacteria.xe2x80x9d
U.S. Pat. No. 4,826,600 teaches a process for altering the pH of a anaerobic system by using methane gas to strip carbon dioxide gas from the system. This process also may employ an xe2x80x9cinertxe2x80x9d gas to aid in the withdrawal of gaseous products. The preferred xe2x80x9cinertxe2x80x9d gas is methane.
U.S. Pat. No. 5,185,079 teaches an anaerobic reactor that removes biogas from the top of a reactor and re-introduces it into the bottom of said reactor. This reference also notes the beneficial effects of applying a vacuum during its settling phase to promote removal of gas bubbles attached to bacterial cell walls.
U.S. Pat. No. 4,372,856 discloses a process wherein a sludge is sparged with methane gas in order to stimulate the growth of anaerobic bacteria and, thus, greater production of biogas. This reference also teaches that biogas may be stripped of its xe2x80x9cundesiredxe2x80x9d carbon dioxide and hydrogen sulfide components by passing it through a scrubbing liquid prepared by use of ammonia produced by another phase of the overall process. This reference does not teach reduction of H2S.
Use of propane as a gas stripping agent in anaerobic digesters also is noted in certain trade literature. For example, the January 1996 issue of the trade magazine Butane-Propane News (p. 32) reports that propane can be bubbled through anaerobic digesters to accelerate their reaction rates. The mode of propane injection is not disclosed in this article. Applicant, however, has personal knowledge that the propane used in this process was injected into the biomass system on a substantially continuous basis.
The trade literature also has recognized that some trace amounts ( less than 0.1%) of propane are produced by large scale (municipal) anaerobic digester systems. For example, a leading trade-oriented publication in this area: Methane Production From Waste Organic Matter, by Stafford et al., CRC Press, Inc., Boca Raton, Florida (1980) (xe2x80x9cthe Stafford referencexe2x80x9d) states (on page 114) that some xe2x80x9csmall quantitiesxe2x80x9d of propane may be formed by xe2x80x9cpolymerizationxe2x80x9d of methane in an anaerobic digester. It also might be noted that this reference also states that, as far as biogas make up is concerned, anaerobic digesters of this kind usually make 60-70% methane and 30-40% CO2 along with rather small amounts of H2S and H2. These statements imply that any other gases (e.g., propane) that may be produced by such anaerobic processes are produced in only trace amounts (e.g.,  less than 0.1%). Applicant has personally measured the propane content of several such anaerobic digesters prior to the introduction of any external propane source and found that their propane concentrations to be  less than 0.1%.
These academic, patent and trade references indicate that much work has been done to improve the performance of anaerobic digestersxe2x80x94and, indeed, much has been accomplished. However, further improvements in this art are still being sought on many fronts and are always welcome when, in fact, achieved. To this end, applicant""s present processes improve the operating performance of anaerobic biomass systems in general, and large scale municipal waste water treatment plants in particular, through use of propane or intermittent injections of a propane-containing gas.
The inventive concepts of this patent disclosure revolve around the fact that, even though the prior art teaches injection of propane gas into biomass systems in order to strip waste product gases from the exterior of anaerobic bacteria cell walls, the full beneficial value of propane with respect to the metabolic processes of such cells has not been achieved. Indeed, it might even be said that, heretofore, the nutrient properties of propane and/or the utilization of propane in the biological processes of anaerobic bacteria have not been recognized.
Applicant believes that the best evidence for the proposition that intermittently injected propane is acting as a nutrient (or taking part in some other, as yet unknown, biological process) in applicant""s process is a well documented lack of mass balance with respect to the injected propane. Applicant has repeatedly found that up to 50% of the injected propane cannot be accounted for in mass balance calculations conducted with respect to processes wherein the propane was introduced on an intermittent basis.
The beneficial effects of the hereindescribed processes are achieved through intermittent, as opposed to continuous, introduction of propane into anaerobic biomass systems. These beneficial effects follow from the fact that, in those periods wherein propane is not introduced, the biomass bacteria have an opportunity to take up the propane through their cell walls and/or to have the propane metabolically interact with the bacteria in some, as yet undefined, manner. Possibly the cell walls serve to better attract the propane under these conditions, and, hence, give it a better opportunity to penetrate the bacteria cell walls and serve as a bacterial nutrient especially for those bacteria involved in the formation of H2S. Conversely, during those periods when the propane is introduced into an anaerobic biomass system, portions of that propane serve primarily as a stripper gas. Thus, applicant""s processes improve upon prior art use of propane in biomass systems by aiding those metabolic processes carried out within the interior of the cell walls, while still dealing with those problems associated with the inhibitory effects associated with methane, carbon dioxide, hydrogen sulfide and hydrogen bubble build up on the exterior surfaces of those cell walls.
Applicant also has found that the improved efficiencies of the hereindescribed processes (relative to prior art processes wherein propane is introduced more or less continuously) result, at least in part, from the fact that H2 levels in prior art processes (wherein stripper gases are continuously introduced) are reduced to levels that lead to inhibition of CH4 formation by biochemical conversion of H2 and CO2 to CH4 within the cell walls. Applicant has postulated that intermittent or cyclical addition of propane gives a biomass an opportunity to build up its H2 levels. Apparently, this circumstance facilitates better conversion of H2 and CO2 to CH4. Applicant""s experimental work also suggests that, even during periods when no propane is injected, minor stripping of the H2, CO2, and CH4 may still occur (at a diminished rate) as a result of residual dissolved propane coming out of solution and dissolving gas molecules adhering to the outer cell walls of the biomass bacteria.
Conversely, during periods of propane injection, H2, CO2, CH4, etc. are more readily stripped off the exterior surfaces of the bacteria cell walls as well as the outer surfaces of any volatile solids present in the digester system. Thereafter, they are removed from the digester liquid. Applicant is of the further opinion that retention of CH4 and CO2 on the surfaces of volatile solid particles may impede attachment of bacteria on to the surfaces of volatile solids by blocking favorable attachment sites on said solids.
The above-noted decreases in H2S production by applicant""s processes (e.g., decreases of almost 50%) also can be extremely beneficial from a purely economic point of view. This follows from the fact that current H2S treatment processes usually involve addition of FeCl2 or FeCl3 to large scale waste water treatment plants. For example, FeCl3 costs of over $55,000 per year are typically incurred by waste water treatment plants of about 25 million gallon per day (mg/d) capacities. Tests conducted at the Greeley, Colorado Waste Water Treatment Plant, data showed that H2S production can be decreased by about 50% through intermittent injection of propane. Applicant has also found that the intermittent propane injection processes of this patent disclosure also result in increased alkalinity of the biomass and increased biological conversion of certain hard to digest hydrocarbons whose degradation is aided by the presence of propane in the system.
Be all that as it may, applicant has found that the prior art practice of injecting propane into biomass systems on a substantially xe2x80x9ccontinuousxe2x80x9d basis in order to perform a waste product gas stripping function is not conducive to anaerobic bacteria cell take up of propane as a nutrient. More importantly, applicant has found that when propane is introduced into anaerobic biomass systems on an intermittent basis the operating efficiency of the digestion process is greatly improved. Applicant also has determined that intermittent introduction of propane results in other desired attributes for such biomass systems e.g., increased alkalinity and decreased H2S levels.
For the purposes of this patent disclosure, the expression xe2x80x9cintermittentxe2x80x9d basis should be taken to mean that in any given period of time in which an anaerobic digester is operating, propane introduction takes place for about 20% to about 80% of that given period of time. Preferably, the propane injection is continuous during the 20% to 80% subperiod of the given period. This requirement for a 20% to 80% subperiod wherein propane is injected, implies that during another 80% to 20% subperiod of that given period of time, propane is not introduced into the anaerobic digester.
In the more preferred embodiments of this invention, a first time period (in which propane is introduced into the digester and then not introduced into the digester) is followed by a second time period wherein propane introduction again takes place for 20% to 80% of that second time period and does not take place for 80% to 20% of that second time period. In certain even more preferred embodiments of this invention, the second period (of propane injection and non-injection) is followed by third, fourth, fifth, etc. time periods having comparable subperiods wherein propane is injected and then not injected. The summation of these consecutive periods can be termed an overall process period (xe2x80x9coverall process periodxe2x80x9d). Here again, in the case where a series of such consecutive time periods are used, it is preferred that the propane be injected continuously during each of the 20% to 80% subperiods wherein the propane is injected.
In the more preferred embodiments of this invention, there will be from 2 to about 100 consecutive periods wherein propane is injected for about 20% to about 80% (and not injected for about 80% to about 20%) of each of said consecutive periods. Intermittent propane injection programs comprised of about 10 to about 50 consecutive periods (each comprising a period of propane injection and a period of non-injection) are even more highly preferred. In other words, these 2 to about 100 (and preferably 10 to about 50) consecutive periods constitute an overall process period.
Those skilled in this art also will appreciate that most large scale, waste water treatment plants operate on a more or less continuous basis. Thus, when the hereindescribed processes are being used in the context of such a continuous operation, the length of applicant""s overall process period will be defined by the summation of the lengths of time for each of the consecutive periods in which propane is injected intermittently during the course of the overall continuous operation.
Next it should be noted that any given time period (wherein propane is injected for a 20% to 80% subperiod, and not injected for a 80% to 20% subperiod), will preferably be from about 30 minutes to about 30 days in length. In certain preferred embodiments of this invention, e.g., operation of municipal waste water treatment plants having capacities of from about 0.5 million gallons per day (mgd) to about 300 mgd, the given time periods (over which propane is injected and not injected) will preferably range from about 4 hours to about 12 hours over an overall process period of at least 1 day. Overall process periods of from 2 to about 365 days duration are even more preferred in the context of large scale (e.g., 10 mgd) municipal waste water treatment plants.
Next, it should be noted that each succeeding time period in any given overall process period can be of the same length, or of a different length, relative to the length of a preceding time period of an overall process period. By way of example only, suppose a given overall process period is comprised of 5 consecutive time periods which are each 8 hours long. Thus, according to applicant""s formula, in the first eight hour period, the propane will be injected from about 1.6 hours (20% of 8 hours) to about 6.4 hours (80% of 8 hours). Likewise, in the second eight hour time period, the propane will be injected for about 1.6 hours to 6.4 hours and so on for the third, fourth and fifth time periods. Similarly, in an overall process comprised of a series of 24 hour periods, propane will be injected for a subperiod ranging from 4.8 to 19.2 hours with no propane being injected for a subperiod ranging from 19.2 hours to 4.8 hours of each of the 24 hour periods that constitute the overall process period.
In other embodiments of this invention, the succeeding time periods in an overall process will not be of equal duration. That is to say that each succeeding time period within an overall process period may be substantially longer than, or shorter than, the preceding period. Again, by way of example only, suppose an overall process period of 10 hours total duration is comprised of four consecutive time periods that respectively have durations of one hour, two hours, three hours and four hours. Thus, in the first period (of one hour duration) the propane will be injected for about 12 minutes (20% of 60 minutes) to about 48 minutes (80% of 60 minutes). In the second period (of two hours duration) the propane will be injected for about 24 minutes (20% of 120 minutes) to about 96 minutes (80% of 120 minutes). Following this formula, the propane will be injected for about 36 minutes (20% of 180 minutes) to about 144 minutes (80% of 180 minutes) in the third period and about 48 minutes (20% of 240 minutes) to about 192 minutes (80% of 240 minutes) in the fourth period. In some preferred embodiments of this invention wherein at least five consecutive time periods are employed, at least 20% of said five consecutive periods will be of a different duration (e.g., 1 out of 5 in the case of 5 consecutive time periods).
Next, it should be noted that in one particularly preferred embodiment of this invention, addition of propane during peak heating demand periods is preferred. Typically, in a large scale digester (e.g., 10 mgd), such periods of peak propane demand occur during relatively cooler evening periodsxe2x80x94or during peak electrical load periods when heat loss from such digesters is usually greater due to cooler evening temperatures. Thus, one particularly preferred embodiment of this invention involves introducing propane into such large scale plants during non-daylight hours of an overall process period comprised of a series of 12 hour time periods.
Next, it should be noted that the hereindescribed processes involve injection of propane (or a mixture of gases such as air, nitrogen, methane, etc. that contains a propane component). It also should be noted that the primary source of the propane in applicant""s processes is not propane obtained from the biogas produced by the process itself. Indeed, applicant has found that very little propane is naturally produced by anaerobic biomass processes in general. Even less is produced by applicant""s intermittent propane injection processes. For example, in a series of tests conducted at the Loveland Wastewater Treatment Plant, Loveland, Colorado, applicant determined that the natural level of propane level of this particular plant""s biogas (i.e., before introduction of propane according to the teachings of this patent disclosure) was only 0.02 volume percent of the biogas. Thus, in the case of the Loveland, Colorado plant, the extraneous propane introduced into this plant according to applicant""s processes, should be intermittently introduced in concentrations such that the biogas product contains more than 0.02 volume percent propane. In any case, in the more preferred embodiments of this invention at least 98% of the propane introduced into those biomass systems operated according to the teachings of this patent disclosure will be obtained from a source other than the biomass system itself.
As a practical matter, however, applicant has determined that the best results for the hereindescribed processes are achieved when propane is intermittently introduced in amounts such that the ratio of propane in the biogas produced is raised from its natural level (e.g., 0.02 volume percent in the case of the Loveland Plant) to about 0.1 to about 5.0 volume percent. Biogas propane concentrations of about 0.5 to about 2.5 volume percent are more preferred. The most preferred propane/biogas concentration is about 1.5 volume percent. Biogas propane concentrations greater than about 5.0 volume percent are not preferred.
Applicant also has found that particularly good biodigestion results are obtained in large scale biomass systems when the concentration of propane gas in the gas injected into the biodigester is about 0.6 to about 4.0 volume percent of the injected gas (e.g., a gas mixture of propane, nitrogen, hydrogen, etc.). Injection of propane in such concentrations will generally produce a biogas product having a propane concentration of from about 0.3 to 2.5 volume percent of the biogas product, Applicant also has found that particularly good results are obtained when the concentration of the propane gas injected into large scale plants is about two and one half times the desired concentration of that biodigester""s biogas product. For example if the desired concentration in the biodigester""s biogas product is 1.0 volume percent, the concentration of the propane injected into the digester is preferably from about 2.0 to 2.5 volume percent.
Next, it should be noted that the propane-containing gas mixture (from whatever xe2x80x9cexternalxe2x80x9d propane source that is being employed) will preferably contain more than about one volume percent propane, and more preferably will contain more than five volume percent propane, and most preferably will contain more than fifty volume percent propane. Bottled propane, liquid propane gas, natural gas (LPG), industrial xe2x80x9cby productxe2x80x9d propane-containing gases and the like all may be employed, in applicant""s processes. Mixtures of gases from such sources also may be employed. Such gases can be injected, sparged, etc. in ways known to those skilled in this art. Those skilled in this art also will appreciate that if bottled, liquid propane is injected into a liquid system it will xe2x80x9cflashxe2x80x9d into a gas form upon being injected into a liquid system.
Applicant also has found that when propane is introduced intermittently, the overall propane addition can be significantly lowered without sacrificing process efficiency relative to otherwise comparable biomass processes wherein propane is introduced on a substantially continuous basis. Indeed, applicant has found that, in most cases, overall digester efficiency is usually raised when the decreased amounts of propane employed in the present processes are injected on an intermittent basis. By way of a concrete example of this, applicant established that the total amounts of propane used in certain prior art anaerobic processes wherein the propane is employed as a stripper gas (e.g., such as the amounts used in the processes disclosed in the ""890 patent wherein propane is introduced continuously in 3% to 15% concentrations as a stripper gas) can be cut by at least about 60% percent without sacrificing the operating efficiency of the digester unit. For example, applicant found that the overall operating efficiency of a process wherein only 2 volume percent propane is intermittently introduced can be as effective as a process wherein propane is continuously introduced at 5 volume percent. This decreased propane requirement represents a significant economic savings. Applicant also has found that introduction of as little as 2.0 volume percent propane, on an intermittent basis, results in a process that has as little as 0.13% propane concentration in its biogas product. That is to say that under these conditions most of the propane was utilized by the bacteria in the biomass system.
Addition of propane to the Greeley anaerobic digester during a first test period was carried out in an intermittent manner (12 hours of propane addition followed by 12 hours of no propane addition) in propane concentrations of 1.8-2.5% by volume. During that first test period, the H2S concentration was found to decrease from 900 ppm (volume H2S/volume Biogas) to 450 ppm in the biogas. During a second test period no propane was added and the H2S concentration again increased to 900 ppm. Thereafter, during a third test period, propane addition was again initiated, but in concentrations of 0.6-0.8 volume percent, in 12 hour cycles, e.g., 12 hours on, 12 hours off. During this third test period the H2S concentration was found to decrease to a 800-850 ppm range. Based upon these observations the following conclusions can be drawn: (1) intermittent addition of propane results in a decrease in H2S concentration in the produced biogas and (2) a decrease in H2S concentration appears to be inversely related to the concentration of propane introduced into the digester; higher concentrations is of introduced propane ( greater than 1.8%) resulted in approximately a 50% lowering of the H2S in the biogas, whereas lower concentrations of propane (0.6-0.8%) resulted in only a 5-12% lowering of the H2S in the biogas. Thus, the effectiveness of applicant""s intermittent propane injection methods also can be measured in terms of decreased H2S concentration in the biogas products of said injection methods.